Basic Soldering Guide

This written
guide will help beginners and novices to obtain effective results when
soldering electronic components. If you have little or no experience of
using a soldering iron, then we recommend that you practice your soldering
technique on some fresh surplus components and clean stripboard
(protoboard), before experimenting with a proper constructional project.
This will help you to avoid the risk of disappointment when you start to
assemble your first prototypes. If you've never soldered before, then read
on!

The most fundamental skill needed
to assemble any electronic project is that of soldering. It takes some
practice to make the perfect joint, but, like riding a bicycle, once
learned is never forgotten! The idea is simple: to join electrical parts
together to form an electrical connection, using a molten mixture of lead
and tin (solder) with a soldering iron. A large range of soldering irons
is available -- which one is suitable for you depends on your budget and
how serious your interest in electronics is.

Electronics catalogues
often include a selection of well-known brands of soldering iron.
Excellent British-made ones include the universally popular Antex, Adcola
and Litesold makes. Other popular brands include those made by Weller and
Ungar. A very basic mains electric soldering iron can cost from under 5 UK
Pounds (8 US Dollars), but you can expect a reasonable model to be
approximately 10 to 12 UKP (16 to 20 US Dollars), and it's quite possible
to spend into three figures on a "soldering station" if you're really
serious! You can check suppliers' catalogues for some typical types of
iron. Certain factors you need to bear in mind
include:

Voltage: Most irons run from the mains at 240V (110V in the US). However,
low voltage types (e.g.12V or 24V) generally form part of a "soldering
station," and are designed to be used with a special controller made by
the same manufacturer.

Wattage: Typically, soldering irons may have a power rating of between
15-25 watts or so, which is fine for most work. A higher wattage does not
mean that the iron runs hotter -- it simply means that there is more power
in reserve for coping with larger joints. This also depends partly on the
design of the "bit" (the tip of the iron). Consider a higher wattage iron
simply as being more "unstoppable" when it comes to heavier-duty work,
because it won't cool down so quickly.

Temperature Control: The simplest and cheapest types don't
have any form of temperature regulation. Simply plug them in and switch
them on! Thermal regulation is "designed in" (by physics, not
electronics!). These irons may be described as "thermally balanced" so
that they have some degree of temperature "matching," but their output
will otherwise not be controlled. Unregulated irons form an ideal general
purpose iron for most users, and they generally cope well with printed
circuit board soldering and general interwiring. Most of these "miniature"
types of iron will be of little use when attempting to solder large joints
(e.g. very large terminals or very thick wires) because the component
being soldered will "sink" heat away from the tip of the iron, cooling it
down too much. (This is where a higher wattage comes in useful.)

A
proper temperature-controlled iron will be quite a lot more expensive --
retailing at say 40 UKP (60 USD) or more. This type of iron will have some
form of built-in thermostatic control, to ensure that the temperature of
the bit (the tip of the iron) is maintained at a fixed level (within
limits). This is desirable, especially during more frequent use, since it
helps to ensure that the temperature does not "overshoot" in between
times, and also guarantees that the output will be relatively stable. Some
irons have a bimetallic strip thermostat built into the handle which gives
an audible "click" in use: other types use all-electronic controllers, and
some may be adjustable using a screwdriver.

Yet more expensive
still, soldering stations cost from 70 UKP (115 USD) upwards (the iron
maybe sold separately, so you can pick the type you prefer). Soldering
stations consist of a complete bench-top control unit into which a special
low-voltage soldering iron is plugged. Some versions might have a built-in
digital temperature readout, and will have a control knob to enable you to
vary the setting. The temperature could be boosted for soldering larger
joints, for example, or for using higher melting-point solders (e.g.
silver solder). These are designed for the most discerning users, or for
continuous production line and professional use. The best stations have
irons which are well balanced, with comfort-grip handles which remain cool
all day. A thermocouple will be built into the tip or shaft, which
monitors temperature.

Anti-Static Protection: If you're interested in soldering a lot of
static-sensitive parts (e.g. CMOS chips or MOSFET transistors), more
advanced and expensive soldering iron stations use
static-dissipativematerials in their construction to ensure that
static does not build up on the iron itself. You may see these listed as
"ESD safe" (electrostatic discharge proof). The cheapest irons won't
necessarily be ESD-safe but never the less will still probably perform
perfectly well in most hobby or educational applications if you take the
usual anti-static precautions when handling the components. The tip would
need to be well earthed (grounded) in these
circumstances.

Bits:
It's useful to have a small selection of manufacturer's bits (soldering
iron tips) available with different diameters or shapes, which can be
changed depending on the type of work in hand. You'll probably find that
you become accustomed to, and work best with, a particular shape of tip.
Often, tips are iron-coated to preserve their life, or they may be
bright-plated instead. Copper tips are seldom seen these
days.

Spare
Parts: it's nice to
know that spare parts may be available, so if the element blows, you don't
need to replace the entire iron. This is especially so with expensive
irons. Check through some of the larger mail-order
catalogues.

Gas-Powered Irons: You will occasionally see gas-powered soldering irons
which use butane rather than the mains electrical supply to operate. They
have a catalytic element which, once warmed up, continues to glow hot when
gas passes over them. Service engineers use them for working on repairs
where there may be no power available, or where a joint is tricky to reach
with a normal iron, so they are really for occasional "on the spot" use
for quick repairs, rather than for mainstream construction or assembly
work.

And
Finally: A solder
gun is a pistol-shaped iron, typically running at 100W or more, and is
completely unsuitable for soldering modern electronic components: they're
too hot, heavy and unwieldy for micro-electronics use. Plumbing,
maybe..!

Soldering irons are best used along with a heat-resistant
bench-type holder, so that the hot iron can be safely parked in between
use. Soldering stations already have this feature, otherwise a separate
soldering iron stand is essential, preferably one with a holder for
tip-cleaning sponges. Now let's look at how to use soldering irons
properly, and how to put things right when a joint goes wrong
......

How to Solder

Topics in this section include: Quick Summary Guide, Cleanliness of
Components, Temperature, Time, and Amount.

Quick Summary Guide:
Turning to the
actual techniques of soldering, firstly it's best to secure the work
somehow so that it doesn't move during soldering and affect your accuracy.
In the case of a printed circuit board, various holding frames are fairly
popular especially with densely populated boards. The idea with a holding
frame is to insert all the parts on one side (this may be referred to as
"stuffing" or "populating" the board), hold them in place with a special
foam pad to prevent them from falling out, turn the board over, and then
snip off the wires with cutters before making the joints. The frame saves
an awful lot of turning the board over and back again, especially with
large boards. Other parts could be held firm in a modeller's small vice,
for example.

Solder joints may need to possess some degree of
mechanical strength in some cases, especially with wires soldered to, say,
potentiometer or switch tags, and this means that the wire should be
looped through the tag and secured before solder is applied. The down side
is that it is more difficult to desolder the joint (see later) and remove
the wire afterwards, if required. Otherwise, in the case of an ordinary
circuit board, components' wires are bent to fit through the board,
inserted flush against the board's surface, splayed outwards a little so
that the part grips the board, and then soldered.

In my view --
opinions vary -- it's generally better to snip the surplus wires leads off
first, to make the joint more accessible and avoid applying a mechanical
shock to the printed circuit board (PCB) joint. However, in the case
ofsemiconductors, I often tend to leave the snipping until after the
joint has been made, since the excess wire will help to sink away some of
the heat from the semiconductor junction. Integrated circuits can either
be soldered directly into place if you are confident enough, or better,
use a dual-in-line socket to prevent heat damage. The chip can then be
swapped out if needed.

Parts which become hot in operation (e.g.
some resistors), are raised above the board slightly to allow air to
circulate. Some components, especially large electrolytic capacitors, may
require a mounting clip to be screwed down to the board first, otherwise
the part may eventually break off due to vibration.

The perfectly
soldered joint will be nice and shiny looking, and will prove reliable in
service. I would say that the key factors affecting the quality of the
joint are:

o) Cleanlinesso)
Temperatureo) Durationo) Adequate solder
coverage

A little effort spent now in
soldering the perfect joint may save you -- or somebody else -- a
considerable amount of time in troubleshooting a defective joint in the
future. The basic principles are as follows.

Really Clean: Firstly, and without exception, all parts
-- including the iron tip itself -- must be clean and free from
contamination. Solder just will not "take" to dirty parts! Old components
or copper board can be notoriously difficult to solder, because of the
layer of oxidation which builds up on the surface of the leads. This
repels the molten solder and this will soon be evident because the solder
will "bead" into globules, going everywhere except where you need it. Dirt
is the enemy of a good quality soldered joint!

Hence, it is an
absolute necessity to ensure that parts are free from grease, oxidation
and other contamination. In the case of old resistors or capacitors, for
example, where the leads have started to oxidise, use a small hand-held
file or perhaps scrape a knife blade or rub a fine emery cloth over them
to reveal fresh metal underneath. Stripboard and copper printed circuit
board will generally oxidise after a few months, especially if it has been
fingerprinted, and the copper strips can be cleaned using an abrasive
rubber block, like an aggressive eraser, to reveal fresh shiny copper
underneath.

Also available is a fibre-glass filament brush, which
is used propelling-pencil-like to remove any surface contamination. These
tend to produce tiny particles which are highly irritating to skin, so
avoid accidental contact with any debris. Afterwards, a wipe with a rag
soaked in cleaning solvent will remove most grease marks and fingerprints.
After preparing the surfaces, avoid touching the parts afterwards if at
all possible.

Another side effect of having dirty surfaces is the
tendency for people to want to apply more heat in an attempt to "force the
solder to take." This will often do more harm than good because it may not
be possible to burn off any contaminants anyway, and the component may be
overheated. In the case of semiconductors, temperature is quite critical
and they may be harmed by applying such excessive heat.

Before
using the iron to make a joint, it should be "tinned" (coated with solder)
by applying a few millimetres of solder, then wiped on a damp sponge
preparing it for use: you should always do thisimmediately with a new
bit, anyway. Personally, I always re-apply a very small amount of solder
again, mainly to improve the thermal contact between the iron and the
joint, so that the solder will flow more quickly and easily. It's
sometimes better to tin larger parts as well before making the joint
itself, but it isn't generally necessary with PCB work. A worthwhile product is Weller's Tip
Tinner & Cleaner, a small 15 gram tinlet of paste onto which you
dab a hot iron -- the product cleans andtins the iron ready for use.
An equivalent is Adcola Tip-Save.

Normal electronics grade solder
is usually 60% lead - 40% tin, and it contains cores of "flux," which
helps the molten solder to flow more easily over the joint. Flux removes
oxides which arise duringheating, and is seen as a brown fluid
bubbling away on the joint. Acid fluxes (e.g. as used by plumbers) must
never be applied. Other solders are available for specialist work,
including aluminium and silver-solder. Different solder diameters are
produced, too; 20-22 SWG (19-21 AWG) is 0.91-0.71mm diameter and is fine
for most work. Choose 18 SWG (16 AWG) for larger joints requiring more
solder.

Temperature: Another step to successful soldering is to ensure that the
temperature of all the parts is raised to roughly the same level before
applying solder. Imagine, for instance, trying to solder a resistor into
place on a printed circuit board: it's far better to heat both the copper
PCB and the resistor lead at the same time before applying solder, so that
the solder will flow much more readily over the joint. Heating one part
but not the other is far less satisfactory joint, so strive to ensure that
the iron is in contact with all the components first, before touching the
solder to it. The melting point of most solder is in the region of 188°C
(370°F) and the iron tip temperature is typically 330°C to 350°C (626°F to
662°F).

Now is
the time: Next, the
joint should be heated with the bit for just the right amount of time --
during which a short length of solder is applied to the joint. Do not use
the iron to carry molten solder over to the joint! Excessive time will
damage the component and perhaps the circuit board copper foil too! Heat
the joint with the tip of the iron, then continue heating whilst applying
solder, then remove the iron and allow the joint to cool. This should take
only a few seconds, with experience. The heating period depends on the
temperature of your iron and size of the joint -- and larger parts need
more heat than smaller ones -- but some parts (semiconductor diodes,
transistors and integrated circuits), are sensitive to heat and should not
be heated for more than a few seconds. Novices sometimes buy a small
clip-on heat-shunt, which resembles a pair of aluminium tweezers. In the
case of, say, a transistor, the shunt is attached to one of the leads near
to the transistor's body. Any excess heat then diverts up the heat shunt
instead of into the transistor junction, thereby saving the devicefrom
over-heating. Beginners find them reassuring until they've gained more
experience.

Solder Coverage: The final key to a successful solder joint is to apply
an appropriate amount of solder. Too much solder is an unnecessary waste
and may cause short circuits with adjacent joints. Too little and it may
not support the component properly, or may not fully form a working joint.
How much to apply, only really comes with practice. A few millimetres
only, is enough for an "average" PCB joint, (if there is such a
thing).

How to Desolder

A
soldered joint which is improperly made will be electrically "noisy,"
unreliable, and is likely to get worse over time. It may even not have
made any electrical connection at all, or could work initially and then
cause the equipment to fail at a later date! It can be hard to judge the
quality of a solder joint purely by appearances, because you cannot say
how the joint actually formed on the inside, but by following the
guidelines there is no reason why you should not obtain perfect
results.

A joint which is poorly formed is often called a "dry
joint." Such a joint usually results from dirt or grease preventing the
solder from melting onto the parts properly, and is often noticeable
because of the tendency of the solder not to "spread," but to form beads
or globules instead, perhaps partially. Alternatively, if it seems to take
an inordinately long time for the solder to spread, this is another sign
of possible dirt and that the joint may potentially be a dry
one.

There will undoubtedly come a time when you need to remove the
solder from a joint: possibly to replace a faulty component or fix a dry
joint. The usual way is to use a desoldering pump which works like a small
spring-loaded bicycle pump, only in reverse! (More demanding users using
CMOS devices might need a pump which is ESD safe.) A spring-loaded plunger
is released at the push of a button and the molten solder is then drawn up
into the pump. It may take one or two attempts to clean up a joint this
way, but a small desoldering pump is an invaluable tool especially for PCB
work.

Sometimes, it's effective to actually add more solder and
then desolder the whole lot with a pump, if the solder is particularly
awkward to remove. Care is needed, though, to ensure that the boards and
parts are not damaged by excessive heat; the pumps themselves have a
P.T.F.E. nozzle which is heat proof but may need replacing
occasionally.

An excellent alternative to a pump is to use
desoldering braid, including the famous American "Soder-Wick" (sic) or
Adcola "TISA-Wick" which are packaged in small dispenser reels. This
product is a specially treated fine copper braid which draws molten solder
up into the braid where it solidifies. The best way is to use the tip of
the hot iron to press a short length of braid down onto the joint to be
desoldered. The iron will subsequently melt the solder, which will be
drawn up into the braid. Take extreme care to ensure that you don't allow
the solder to cool with the braid adhering to the work, or you run the
risk of damaging PCB copper tracks when you attempt to pull the braid off
the joint.

I recommend buying a small reel of de-soldering braid,
especially for larger or difficult joints which would take several
attempts with a pump. It is surprisingly effective, especially on
difficult joints where a desoldering pump may prove a
struggle.

Summary

Here's a summary of how to make the perfect solder joint:

All parts must be
clean and free from dirt and grease.

Try to secure the
work firmly.

"Tin" the iron tip
with a small amount of solder. Do this immediately, with new tips being
used for the first time.

Clean the tip of the
hot soldering iron on a damp sponge (many people then add a tiny amount
of fresh solder to the cleansed tip).

Heat all parts of
the joint with the iron for under a second or so.

Continue heating,
then apply sufficient solder only, to form an adequate joint.

Remove and return
the iron safely to its stand.

It only takes two or
three seconds at most, to solder the average PCB joint.

Do not move parts
until the solder has cooled.

Troubleshooting Guide

The solder won't "take": If grease or dirt are present, desolder and clean up
the parts. Or, the material may simply not be suitable for soldering with
lead/tin solder.

The joint is crystalline
or grainy-looking:
The parts forming the joint may have been moved before being allowed to
cool, or the joint was not heated adequately (too small an iron or too
large a joint).

The solder joint forms a
"spike": The joint
was probably overheated, burning away the flux.

First Aid

If you are unlucky
enough to receive burns which require treatment, here's what to
do:

Immediately cool the affected
area with cold running water, ice, or even frozen peas, for ten
minutes.

Remove any rings etc. before
swelling starts.

Apply a sterile dressing to
protect against infection.

Do not apply lotions,
ointments etc., nor prick any blisters which form later.

Seek professional medical
advice where necessary.

Photo Gallery

Soldering is
a delicate manual skill which only comes with practice. Remember that your
ability to solder effectively will determine directly how well the
prototype or product functions during itslifespan. Poor soldering can
be an expensive business - causing product failure and downtime,
engineer's maintenance time and customer dissatisfaction. At hobbyist
level, bad soldering technique can be a cause of major disappointment
which damages your confidence. It needn't be like that: soldering is
really easy to learn, and like learning to ride a bike, once mastered is
never forgotten! These ten photos illustrate the basic steps in making a
perfect solder joint on a PCB. If you're a beginner, our advice is that
it's best to practice your soldering technique using some clean, new parts
with perhaps some new stripboard (protoboard). Be sure to avoid using old,
dirty parts; these can be difficult if not impossible to solder.

Enjoy! -- Alan
Winstanley.

Boards must be clean to begin
with, especially if they're not previously "tinned" with solder. Clean the
copper tracks using e.g. an abrasive rubber block.

Clean the
iron "bit" (tip) using a damp sponge. The iron featured here is an Ungar
Concept 2100 Soldering Station.

A useful
product is Multicore's Tip Tinner Cleaner (TTC) - a 15 gramme tin of
special paste which cleans and "tins" the iron, in one go.

Insert the
components and splay the leads so that the part is held in
place.

It's usually
best to snip the wires to length prior to soldering. This helps prevent
transmitting mechanical shocks to the copper foil.

Apply a
clean iron tip to the copper and the lead, in order to heat both items at
the same time.

Continue
heating and apply a few millimetres of solder. Remove the iron and allow
the solder joint to cool naturally.

It only
takes a second or two, to make the perfect joint, which should be nice and
shiny. Check the Guide for troubleshooting help.

An example
of a "dry" joint - the solder failed to flow, and instead beaded to form
globules around the wire.

"Solder
Wick" is a cheap and very effective way of desoldering a joint. Take care
not to overheat the board. Alternatively, use a desoldering
pump.

Every care has been taken
to ensure that the information and guidance given is accurate and
reliable, but since conditions of use are beyond our control, no legal
liability or consequential claims will beaccepted for any errors
herein.

The British mains voltage supply is 230V a.c. -- you should
amend ratings for local conditions.

For any interested photographers:
the photographs were taken by the author using a Minolta X-700 SLR with
50mm Minolta MC manual-focus macro lens at f11-16, coupled to a Minolta
Auto 80PX macro ring flash gun. Film was Kodak Gold 200. The prints were
scanned using an HP Scanjet 4C flatbed scanner, and enhanced using JASC
Paintshop Pro 4.1.
V1.1